1. Field of the Invention
This invention relates to polypropylene film, specifically to mono-axially oriented polypropylene film with sodium benzoate as a nucleating agent to reduce shrinkage and increase stretchability.
2. Description of the Prior Art
Polypropylene copolymers and homopolymers are produced in numerous grades and formulations. Among the outstanding properties of polypropylene are low density, high melting point, high tensile strength, rigidity, stress crack resistance, abrasion resistance, low creep and a surface which is highly resistance to chemical attack.
Polypropylene film (usually 0.5 to 1.0 mil in thickness) is made by tubular and casting film extrusion methods. Two forms of polypropylene film are produced, oriented and unoriented. Unoriented film has good transparency, tensile strength, machinability, electrical properties, high gloss, resistant to grease and oil, heat resistance and can be printed, coated and heat-sealed. Orienting polypropylene film improves some of its properties, particularly tensile strength, stiffness, moisture- and gas-barrier properties, grease and oil resistance, low-temperature impact strength, abrasion resistance, optical properties and heat shrinkability.
In a tape orienting process, amorphous orientation and destruction of crystal structure in cold-drawing lead to a thermodynamically unstable system which, given some time, tend toward some equilibrium. Given the opportunity, the strained molecular segments in this unstable system will try to rearrange themselves in a state of lowest potential energy. The process results in tape shrinkage and, consequently, tape dimension instability.
The use of inorganic fillers to improve the mechanical properties of polyolefin films is known. Using very small amounts (below 0.5 wt %) a significant improvement in transparency is attained. This improvement is attributed to the formation of smaller, more numerous spherulites as the polymer solidifies from the molten state. Accompanying this change in morphology is a marked increase in crystallization rate and a higher crystallization temperature. In addition to the improved optical properties, nucleation results in better processing characteristics and improved strength.
Certain inorganic fillers and organic compounds serve as nucleating agents for polyethylene, polyethylene terephthalate (PET), polyamides, polypropylene and polybutylene. Addition of these nucleating agents to olefin polymers provides an increase in the crystallization temperature and/or an improvement in the optical properties of the polymer. The use of nucleating agents can allow a shorter cycle time and better productivity. However, the use of certain nucleating agents may be limited by costs, processing problems or acceptability in medical or food applications.
Nucleating agents allow the polymer to be crystallized at a higher temperature during processing. Examples of nucleating agents are aromatic carboxylic acids and their metal salts, such as sodium benzoate, aluminum p-tert-butyl benzoate, and aluminum benzoate; metal organophosphates such as di(4-tert-butylphenyl) phosphate; benzylidene sorbitol compounds; talc, polyvinyl Cyclohexane; and organic dicarboxylic acids such as oxalic acid, 1,2-cyclohexanedicarboxylic acid, phathalic acid and malonic acid.
The size of polymer spherulites can be adjusted by incorporating nucleating agents. The use of nucleating agents favors smaller spherulite sizes.
As noted above, sodium benzoate is a known nucleating agent. Japanese Patent Application 80329/1983 discloses a specific method of dispersing sodium benzoate as a nucleating agent in polypropylene to improve transparency. However, as disclosed in U.S. Pat. Nos. 5,135,975 and 5,112,894, sodium benzoate is a higher melting nucleating agent that has dispersion problems. Sodium benzoate has been disclosed in U.S. Pat. No. 5,118,566 as a nucleating agent in a biaxially-oriented film with high mechanical strength.
In the article "Making Microcellular Foams from Crystalline Polymers" by Jonathan S. Colton in Plastics Engineering, Vol. 44, No. 8, page 53 (1988), it was disclosed that small amounts of sodium benzoate in polypropylene enhanced nucleation by providing a large enough number of sites with appropriate surface energy conditions and by reducing the overall surface tension of the polymer. This article recommends an amount of nucleating agent near its solubility limit, typically 1% by weight (ibid., page 55).
Nucleating agents are typically used to stiffen polymer film. While film of formulations using nucleating agents such as sodium benzoate may show an increase in the modulus of elasticity over that for unnucleated film, such formulations, as disclosed in U.S. Pat. No. 5,118,566, do not result in an decrease in shrinkage. In fact, as demonstrated by the data in U.S. Pat. No. 5,118,566, an increase in shrinkage may result.
It would be advantageous to use an additive in uni- or mono-axially oriented film which would reduce shrinkage. Low shrinkage is a highly desirable property for such applications as woven fabrics. It would be preferable if the additive is known to be acceptable as a component for food or medical applications.